Lets begin by looking at the differences between spinal nerves and cranial nerves.

Cranial nerves are vital and important for life. Hence, while spinal nerve nuclei have UMN supply only from the opposite cortex, most cranial nerves have UMN supply from BOTH motor cortices (bilateral cortical representation). This is probably the nature's way of ensuring fail-safe mechanisms.

However, facial nerve is a partial exception to this rule of bilateral cortical representation. The facial nerve nucleus has two anatomically indistinct parts - upper part and lower part. (Please view the attached PowerPoint document created to explain this concept). The lower part is an exception to the above rule and receives UMN fibers only from the opposite motor cortex. Upper part gets from both cortices.

When there is a lesion, for example, in the right internal capsule, (refer to the PowerPoint document), the UMN fibers TO the left facial nucleus are affcted. However, the left facial nucleus continues to receive UMN impulses from the intact right cortical connections and is spared. It is the lower part of the left facial nucleus that is affected, thus causing involvement of the left lower part of the face.

However, when the facial nerve itself is involved, that being the final pathway of neural impulses to the face, the entire half face on that side is paralysed.

Hope this explanation, with the PowerPoint illustrations is clear. Please do not hesitate to ask any further doubts here.

Shashikiran

Hello,
You are right. Doubts arise only after you have understood a topic


For the sake of simplicity, lets consider that there are TWO types of motor fibres in the internal capsule - (i) fibres for the facial nucleus and (ii) fibres going down to the spinal cord to supply the limb muscles.

If you consider the right internal capsule, these two types of fibres, both being UMN fibres have not yet decussated to the left side. The facial fibres there are to supply the left facial nucleus and limb fibres to supply the left upper and lower limbs.

If there is a lesion in the right internal capsule, therefore, the fibres involved are those UMN fibres that supply the left facial nucleus (resulting in left UMN facial palsy) and left sided limbs (resulting the left sided hemiplegia).

In this situation, as both the paralysis, that is, paralysis of 'face' and 'limbs' are on the 'same side of the body', it is called 'ipsilateral hemiplegia'. (But please remember that it is of course contralateral to the side of lesion!)


Now consider a lesion in the left right brainstem, for example at the level of right pons. What do we have at the level of pons? For the sake of simplicity, let us consider that there are TWO things there - (i) the facial nucleus and (ii) UMN fibres going down to the spinal cord to supply the limb muscles - the pyramidal tract.

The facial nucleus, as you are aware, is made of neurons which constitute the lower motor neurons for the facial nerve. Lower motor neurons DO NOT decussate. On the other hand, the UMN fibres of pyramidal tract for the limbs have NOT YET decussated. Decussation takes place at the level of medulla, which is below the pons.

If there is a lesion at the right pons, the involvement of the right facial nucleus results in right LMN facial palsy. Involvement of the pyramidal tract results in left hemiplegia as the fibres decussate to the left at medullary level.

As the facial palsy and the limb palsy are on 'different sides of the body', this type of palsy is also called 'contralateral hemiplegia' or crossed hemiplegia'.

I will post an image soon to make this further easy to understand. Please do not hesitate to reply if there is anything that is still not clear.

Shashikiran

hi sir,


that's the msg sir put... but wat did sir meant by lesion in LEFT BRAINSTEM with an example at the level of RIGHT PONS? why the sides are different?

and.. when we use contralateral or ipsilateral... wat we meant? the site of lesion with the manifestation in the body.. or the manifestation in the different part of body?

and.. where does the facial nerve fibers decussate?

and.. when we say right UMN facial palsy... we referring to the side of lesion or the side of manifestation?? means the lesion occurs at right side causing left lower face afftected.. or there is right sided lower face involvement caused by a left sided lesion?

whenever i have a facial palsy case,i'll need to think the whole pathway to confirm that its UMN or LMN...because i dun wan to memorise that in UMN we'll have lower face involvement.. or in LMN we'll have whole half involvement... is there any easier way... or i have to memorise like that.. or i'll automatically know which side if i prac more?

thanku sir..

hi sir.. thanku sir for the clarification...

now i'll remember.. UPPER face spared in UPPER motor neuron type palsy... so any UMN lesion,the upper face is spared along with UMN features in motor system like hyper reflexia, hypertonia, babinski +ve, absent superficial reflexes..